1. Field of the Invention
Embodiments of the present invention relate generally to disk drives and, more particularly, to a method for iterative real-time determination of wedge offset compensation values for such drives.
2. Description of the Related Art
A disk drive is a data storage device that stores digital data in tracks on the surface of a data storage disk. Data is read from or written to a track of the disk using a transducer, which includes a read element and a write element, that is held close to the track while the disk spins about its center at a substantially constant angular velocity. To properly locate the transducer near the desired track during a read or write operation, a closed-loop servo system is generally implemented. The servo system uses servo data read from a “servo wedge” on the disk surface to align the transducer with the desired track, where the servo data may include the track number as well as “servo bursts” that indicate how far the recording head is from the ideal track center line. The servo data are previously written on the disk surface by the drive itself using a self-servo-writing procedure or by an external device, such as a servo track writer (STW). In either case, an additional factory calibration for each track present on the disk drive may be required to compensate for small errors in the position of the servo bursts written on the disk surface. Because modern disk drives typically include hundreds of thousands of tracks, such factory calibration is a time-consuming part of the manufacturing process.
In an ideal disk drive, the tracks of a disk are non-perturbed circles situated about the center of the disk. As such, each ideal track includes a track centerline that is located at a known constant radius from the disk center. In practice, however, writing non-perturbed circular tracks to a disk is problematic due to imperfections in the media itself and/or in the position control of the device writing the servo bursts caused by mechanical effects, e.g., vibration, bearing defects, inaccuracies in the STW, disk clamp slippage, etc. Thus, the servo bursts that define each track are generally written with an offset from the ideal non-perturbed circular track shape. Positioning errors created by the offset between the real servo burst locations and the ideal track location are known as written-in repeatable runout (WRRO).
Without additional correction to the servo bursts as written to a disk, the non-ideal shape of the tracks as defined by the servo bursts creates two problems. First, the transducer positioning function is made more complicated during read and write operations because the servo system continuously repositions the transducer during track following to keep up with the constantly changing radius of the track centerline as defined by the servo bursts, rather than following the constant radius of an ideal circle. Second, the perturbed shape of these tracks can result in problems such as track misregistration errors during read and write operations and “track squeeze,” i.e., adjacent tracks that are spaced too close together.
Disk drive manufacturers have developed techniques to measure the WRRO for each track of a disk drive and produce compensation values that allow the servo system of the disk drive to substantially ignore the offset between the desired track centerline and the actual position of the servo bursts on the disk surface. The compensation values are typically generated for each servo wedge of each track of a disk drive. A number of schemes are known in the art for producing such compensation values, but each involves controlling the transducer to follow a given track for a number of revolutions while measuring a position error signal (PES) as the transducer passes over the servo bursts. In some schemes, PES is measured over multiple revolutions of the disk, and then an average wedge offset compensation value is calculated for each servo wedge. In other schemes, the wedge offset compensation values are determined with increasing accuracy iteratively over multiple revolutions. In either case, multiple revolutions of the disk are necessary to obtain accurate compensation values for the servo bursts associated with each track, and each additional revolution needed significantly increases the time required for factory calibration of a disk drive to complete. For example, given a disk drive operating at 7200 rpm and having four disk surfaces of 150,000 tracks each, determining wedge offset compensation values for the entire disk can take over five hours when an average of four revolutions is required to obtain accurate compensation values for a single track. This time is doubled for a self-servo writing scheme. Thus, reducing the average number of revolutions per track for obtaining accurate compensation values by just one revolution (e.g., from four to three) would reduce the calibration time of a disk drive by one or more hours.
In light of the above, there is a need in the art for a method of determining wedge offset compensation values for a disk drive that can determine such compensation values in a minimum number of revolutions of a data storage disk.